During integrated circuit device design and manufacture, it may be desirable to determine a device's electrical characteristics through testing procedures. For example, an integrated circuit device may be modeled as a two-port network (e.g., a network having an input port and an output port), and the device's electrical characteristics may be determined by providing excitation signals at one port and measuring either reflected signals at the same port or transmitted signals at the other port. Such testing methods may be used to determine various electrical parameters that characterize the device, such as S-parameters (scattering), Y-parameters (conductance), Z-parameters (resistance), and H-parameters (conductance and resistance). For devices that operate at radio frequencies (RF), S-parameters are generally easier to measure than other parameters.
A typical testing system may include a Vector Network Analyzer (VNA), probes, and coaxial cables connected between the probes and the VNA's ports. Each probe may include multiple probe tips. For example, a ground-signal-ground (GSG) type of probe may include three probe tips (e.g., one tip connected to signal and two tips connected to ground). Conversely, a ground-signal (GS) type of probe may include just two probe tips (e.g., one tip connected to ground and one tip connected to signal). The electrical parameters of a device under test (DUT) may be determined by touching the probes to pads associated with the device's input and output ports, and controlling the VNA to provide excitation signals and measure responsive signals at the DUT ports. The excitation and responsive signals may then be evaluated to determine the parameters.
Desirably, the electrical parameters determined by the testing system represent only the electrical characteristics of the DUT, and not the electrical characteristics of the testing system. However, the VNA, cabling, and probes may contribute significant errors to the test results. Therefore, prior to DUT testing, a calibration procedure typically is performed in order to determine an error model for the testing system. The error model may be derived from measurements taken during exposure of the testing system probes to electrically shorted conditions, open conditions, load conditions, and thru conditions. Once the error model is determined, it may be used to adjust measurements or parameters determined during actual DUT testing, in order to negate the signal effects that are contributed by the testing system.
A VNA calibration procedure may be performed using calibration “standards,” which consist of various conductive patterns fabricated on a semiconductor wafer (e.g., a wafer containing the DUT) or on a separate substrate. For example, FIG. 1 illustrates a conventional set of calibration standards 100, which include a first conductive pattern 102 for testing a shorted condition, a second conductive pattern 103 for testing a load condition, and a third conductive pattern 104 for testing a thru condition. FIG. 1 also illustrates GSG probes 110, 112 contacting each of patterns 102-104. The first conductive pattern 102 is a “short-type” calibration standard, which includes two low-resistance conductive strips 120, 121 oriented in a vertical direction, with respect to FIG. 1. When probes 110, 112 accurately contact the first conductive pattern 102, as illustrated, the tips of each probe 110, 112 are shorted together. The second conductive pattern 103 is a “load-type” calibration standard, which includes probe contact pads 122, each separated by known resistance loads 123 (e.g., 50 ohm loads). When probes 110, 112 accurately contact the probe contact pads 122 of the second conductive pattern 103, as illustrated, a load is presented between adjacent probe tips of each probe 110, 112. Finally, the third conductive pattern 104 is a “thru-type” calibration standard, which includes three low-resistance conductive strips 124, 125, 126 oriented in a horizontal direction, with respect to FIG. 1. When probes 110, 112 accurately contact the third conductive pattern 104, as illustrated, corresponding probe tips of probes 110, 112 are electrically connected together. By performing multiple excitation and measurement procedures using the various calibration standards 100, the electrical characteristics of the testing system may be measured, and an error model for the testing system may be determined.
Although the illustrated and described calibration standards may provide sufficient calibration accuracy in many cases, they may not provide adequate accuracy in others (e.g., when the conductive patterns are fabricated on relatively high dielectric, “electrically thick” substrates). In addition, the conductive patterns may exhibit “end effects,” which may compromise the accuracy of the calibration method.